1. Field of the Invention
The present invention relates to a method of isolating semiconductor laser diodes (hereinafter, referred to as semiconductor LD).
2. Description of the Related Art
As necessity for high-density information recording increases, demand for semiconductor light emitting diodes which emit visible light is increasing. Accordingly, various types of semiconductor LDs which can emit a visible range of laser are being manufactured. Among them, a group III-V nitride semiconductor LD is a direct transition type LD of which the laser oscillation probability is high and which can perform blue laser oscillation. Therefore, the group III-V nitride semiconductor LD is attracting attention.
In general, to manufacture a high-power semiconductor LD of which the threshold current value is low, a semiconductor LD should be cut in such a manner that the cut surface thereof is smooth while the laser emitting surface of the cut semiconductor LD is perpendicular to an oscillation layer.
However, when the laser emitting surface is not perpendicular to the oscillation layer and the surface becomes rough during the process of cutting the semiconductor LD, the brightness of laser is significantly reduced, and laser is diffused on the laser emitting surface.
In this case, the critical current of the semiconductor LD increases.
Now, a conventional method of isolating semiconductor LDs, which relates to the cutting of the semiconductor laser diode, will be described with reference to FIGS. 1 and 2.
First, an n-type GaN-based compound semiconductor layer 4, a GaN-based active layer 6 where laser emission is performed, and a p-type GaN-based compound semiconductor layer 8 are sequentially formed on a sapphire substrate 2. Then, a predetermined etching process is performed to form a plurality of semiconductor LDs of which each has a ridge portion 10 and an n-electrode portion 14.
Subsequently, a p-electrode 16 and an n-electrode 18 are formed on the ridge portion 10 and the n-electrode portion 14, respectively. Then, base cut lines are formed on the bottom surface of the sapphire substrate 2.
The base cut line is formed along a virtual isolation line L for isolating the semiconductor LDs by using a diamond tip.
After the base cut lines are formed on the bottom surface of the sapphire substrate 2, a predetermined force is applied onto the ridge portion 10 on the virtual isolation line L by using a ceramic knife. Then, as shown in FIG. 2, the semiconductor LDs 22 are isolated from each other such that a cleavage plane 20 is formed on each of the semiconductor LDs 22. The cleavage plane 20 is used as a light emitting surface.
To reduce a threshold current Ith and increase power, the cleavage plane 20 should be perpendicular to the active layer 6, and the smooth state of the cleavage plane should be maintained.
In the conventional method, however, the n-type compound semiconductor layer, the active layer, and the p-type compound semiconductor layer, which are formed on the substrate, should be cut. Therefore, after the base cut lines are formed, a force should be applied to the ridge portion 10 on the virtual isolation line L. When the force is applied to the ridge portion 10, the ridge may be damaged, and a crack may occur so as to propagate to the light emitting surface.
Further, since the cross-sectional surface formed along the base cut line formed on the bottom surface of the substrate is not smooth, the rough portion of the bottom surface of the substrate corresponding to the position of the ridge propagates to the ridge portion. Therefore, the surface of the light emitting surface (cleavage plane) becomes rougher.
FIGS. 3 and 4 are SEM (scanning electron microscope) photographs showing a state where the rough portion of the bottom surface of the substrate propagates to the ridge portion such that a stepped portion is formed on the light emitting surface (cleavage plane). FIG. 3 shows the cleavage plane excluding the ridge portion, and FIG. 4 shows the cleavage plane of the ridge region.
As shown in FIGS. 3 and 4, it can be found that a stepped surface referred to as striation is formed on the cut surface of the substrate, that is, the cleavage plane. The striation which propagates from the substrate 110 increases an operation current of elements and degrades an optical characteristic.
In the above-described conventional method, after the base cut lines are formed on the bottom surface of the sapphire substrate 110 on which the semiconductor LDs are not formed, a force is applied to the surface of the substrate 110, on which the semiconductor LDs are formed, to thereby isolating the semiconductor LDs from each other. Therefore, the force is concentrated on the ridge portion such that the ridge is damaged, and a stepped surface (striation) is formed on the cleavage plane.
The damage of the ridge and the light emitting surface degrades the optical characteristic of the semiconductor LDs, such as the shape of spots or an output, as well as the electrical characteristic of the semiconductor LDs, such as a threshold current or driving voltage. Therefore, the yield and reliability of the semiconductor LDs decreases.